By selling an article for Rs. 21, a man lost such that the percentage loss was equal to the costprice. The cost price of the article was
Cost price (CP), Selling price (SP), Loss (L)
Given that SP = 21
Assume CP = X
L = CP - SP
% loss was equal to the CP , $$ \frac{CP - SP}{CP} =CP% $$
Substituting,
$$ \frac{x - 21}{x} = \frac{x}{100} $$
On solving,
$$ x^2 = 100x - 2100 $$
$$ x^2 - 70x - 30x + 2100 = 0 $$
x(x-70) - 30(x - 70) = 0
(x - 30)(x - 70) = 0
x = Rs 30 or Rs 70
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